Billy is a book-only character whose "freaky flaw" is that he's invisible.
Billy was introduced as a sort of practical jokester, using his invisibility to his advantage. However, once his character developed a bit more, he was seen as charming and sweet, going to great lengths for his friends.
Billy is good friends with Candace Carver, as she acts as an advisor to him, and even volunteered to make him "visible". He is also best friends with Frankie Stein, despite their complex romantic past.
Billy was originally interested in Frankie Stein, but stopped when he realized she didn't and wouldn't return the feelings. In Where There's a Wolf, There's a Way, Billy expressed interest in Spectra Vondergeist, a girl he'd known since eighth grade. As of Back And Deader Than Ever, the two are dating.
In the Books
Book 1 - Monster High
Frankie first meets Billy at a RAD meeting, where he shows his support for her cause. He also saves her by carrying her out of the school, and gives her a rose.
Book 2 - The Ghoul Next Door
Billy plays a key role by dressing up like Frankie to get everyone off her trail, but that failed due to Brett's bad timing. He also helps found NUDI (Normies Uncool with Discriminating Idiots).
Book 3 - Where There's a Wolf, There's a Way
Candace decides to use fake tanner, hair dye, etc. to make Billy "visible". Using his new visible persona, he takes Frankie out on a date, thinking she was over Brett after the documentary incident had him labeled as a villain. However, Billy found out the hard way that Frankie still had feelings for him. Billy did the right thing and broke it off with Frankie in a chilvarous manner. All was not lost, though, as Billy met Spectra at Clawdeen's Sassy Sixteen party, and the two started flirting.
Book 4 - Back and Deader Than Ever
Billy and Spectra officially become a couple, though Frankie doesn't seem too happy about it. Not because of jealousy though, she's worried that Spectra's talent for mixed information and gossip will get Billy hurt. During the elections for the representatives of the school for the Teau'Dally, he switches the ballots not wanting Cleo to win and declares Frankie the winner, which she was not happy about. He sincerely apologizes and the two share a (friendly) moment.
- Billy is so far the only male character to have a chapter in the books centered around him.
- As an invisible boy, Billy Phaidin has a resemblance to Invisi Billy, a character set to appear in the conventional Monster High continuities However, the two are nothing alike in build, clothing, personality or manifestation of their invisibility, which suggests that they are not the same character. Time will tell whether the two are the same character or not.
- The name Invisi Billy first appeared in Chapter 9 of "The Ghoul Next Door". While driving Melody & Billy to the hospital to put their plan into action, Candace asks him if she can call him "InvisiBilly".